1. Field of the Invention
The present invention provides a method for treatment or prevention of equine protozoal myeloencephalitis, a condition caused by Sarcocystis spp. for which there is presently no effective treatment.
2.Description of the Related Art
Equine protozoal myeloencephalitis (EPM) was initially observed by J. R. Rooney in 1964. Since then, cases have been reported among native horses in most of the United States as well as in Canada, Mexico, Argentina, Panama, and Brazil. During the 1970's, a protozoan was recognized in histopathologic sections of neural tissues of affected horses resulting in the disease being named "equine protozoal myeloencephalitis," but the actual protozoan was not known. In 1991 Dubey showed that EPM was caused by a Sarcocystis organism. Subsequently the organism was named Sarcocystis neurona because it often develops within neurons.
In 1995, results of polymerase chain reaction (PCR) analysis and work done at the University of Kentucky and at the University of Florida provided strong evidence that the opossum (Didelphis virginiana) is the definitive host of Sarcocystis neurona. This study revealed a 99.67% homology with the opossum sporocyst, Sarcocystis falcatula. It is possible that S. neurona will be re-named to S. falcatula.
Opossums eat almost anything: dead birds, insects, etc. The feces of the opossum represent a rich source of nutrition for wild birds. Wild birds ingest sporocysts by eating opossum feces. The parasite undergoes asexual reproduction in the blood vessels of the liver, lungs and muscles and then encysts in the bird's muscle tissue, without traveling to the central nervous system. When bird tissue is eaten by the opossum, the parasite undergoes sexual reproduction in the intestinal cells, and forms the infective sporocysts, which are passed in the feces. The opossum does not become sick, but may shed the parasites for months.
The opossum is indigenous to North, Central and South America which coincides with the fact that cases of EPM have only been reported in horses that have lived in areas that the opossum inhabits. Infected horses cannot communicate the disease to other horses; infection occurs when the droppings of opossum or wild birds carrying the disease mix with the feed of the horse, and the horse consumes the feed.
S. neurona (falcatula) may aberrantly infect a large number of intermediate hosts such as it does in the horse, with reports of infection in dogs, sheep, cats, mink, raccoons, striped skunks, golden hawks, rhesus monkeys and chickens. In addition to the predator-prey life cycle, there is some speculation there are transport vectors which may play a role in this disease, including cockroach, and wild birds such as pigeon, finch, canary, grosbeak, and budgerigar which form sarcocysts after ingestion of the sporocysts of S. falcatula.
Horses represent an aberrant host of this protozoan. Sporocysts are ingested, but never encyst in the tissues of the horse. Instead, they migrate to the central nervous system, where they continue to undergo asexual reproduction intracellularly in neurons, without forming tissue cysts. Central nervous system (CNS) lesions in the horse are often extensive, and may be microscopic to several centimeters wide. The brainstem and spinal cord are affected most often.
The actual signs of abnormality depend on the location and the extent of colonies. The disease may be "encephalitis" when effects of damage are seen in the brain; it may be "myelitis" when effects of damage are seen in the spine; or may be "encephalomyelitis" or "myeloencephalitis" when both the brain and the spine are affected. Symptoms of EPM may be attributed to either the body's inflammatory reaction (swelling) to the parasites or to actual destruction of nerve structures within the CNS. Sometimes, there is a discrete loss of specific muscle groups (neurogenic atrophy). EPM might also cause abortion in pregnant mares.
Exposure of horses to EPM occurs at an average rate of about 50%, but approaches 80-90% among the racing equine population. Only a relatively small number of infected horses actually develop disease due to parasite damage in the central nervous system.
Most EPM-affected horses are treated with antiprotozoal drugs for at least 4 months, and improve 1-2 grades in neurologic symptoms. The dosage recommendations for the treatment of EPM are pyrimethamine antimalarial medications (1.0 mg/kg daily), in combination with a sulfonamide antibiotic, for a minimum duration of 120 days. This combination causes a sequential blockade of folate metabolism in apicomplexan protozoa. This treatment protocol can be expensive, costing $400/month to $850/month.
This antimalarial/antibiotic treatment does not appear to eliminate the Sarcocystis neurona organism; it merely constrains the organism until the body is able to eliminate it. Some horses are not able to eliminate the organism--they get better because the parasite is inhibited, but they are at risk for further problems if their immune system is compromised again in the future. Recovered horses must be retreated strategically on a frequent basis for the remainder of their lives. Only a small percentage of horses make a complete recovery, some horses do not respond to treatment whatsoever, and a significant number of horses eventually relapse and develop further active EPM.
Recent discussions concerning EPM have led to reports of using tetracyclines in some cases that continue to be cerebral spinal fluid positive (CSF+) even after many months (6 months or longer) with the sulfa/pyrimethamine combination. This use is based upon tetracycline's inhibitory effect on protein synthesis, however, no controlled trials have been performed at this time. The only reported antiprotozoal use of tetracycline was documented in sheep which received 30 mg/kg to treat a different Sarcocystis spp. infection. This an extremely high dosage, and is not recommended for use in horses. It is not clear why using tetracycline, a bacteriostatic agent, would be curative.
In addition to the low success rate, a further problem with the presently available therapy concerns undesirable side effects of the therapy such as anemia, leukopenia, and colitis. Some horses actually get worse during treatment, presumably because the administration of a large dose of medication triggers development of an inflammatory response due to the killing of large numbers of parasite at one time. The spinal canal is a very narrow space with limited room for expansion due to inflammation.
At this time, there is no vaccine available to protect against the disease, though this is currently an area of research.
Accordingly, it is an object of the present invention to provide therapeutic compounds useful in the treatment of animals afflicted with, or at risk of being afflicted with, equine protozoal myeloencephalitis.
It is a further object of the invention to provide therapeutic compounds for treatment of protozoal and helminth infections in horses which therapeutic compounds are well tolerated by horses when administered in therapeutically effective amounts.